In “Why Evolution is True,” evolutionary biologist Jerry A. Coyne of the University of Chicago summarizes the theory of evolution by natural selection as follows:
Life on Earth evolved gradually beginning with one primitive species – perhaps one self-replicating molecule – that lived more than 3.5 billion years ago. It then branched out over time, throwing off many new and diverse species, and the mechanism for most (but not all) of evolutionary change is natural selection.
Thus, the theory of biological evolution has two primary claims. First, that all life on earth evolved from a common ancestor. And second, that the evolution of life is driven by natural processes. This video, by Stated Clearly, explains the evidence that exists for these claims.